The present invention relates to a method for dosing a liquid using a pipette and a syringe, and to a pipette for operating a syringe for dosing a liquid.
The pipettes for operating a syringe discussed here serve for discharging liquid taken up into the syringe in several steps. They are also called dispensers or repeater pipettes. At the lower end of a bar-shaped housing, these pipettes have a fixture for a flange of a cylinder of the syringe, and in the housing a displaceable fixture body with a piston fixture for the upper end portion of a piston rod of a piston of the syringe. The syringe can be inserted with the flange and the end portion of the piston rod through axially oriented openings of the fixtures. The flange and the end portion are held in the fixtures by means for detachable holding, which are for instance configured as spring-tensioned gripping levers. Further, the pipette has means for relocating the fixture body, which permit to draw the piston partially out of the cylinder in order to aspirate liquid into the syringe, and to push it stepwise into the cylinder for stepwise discharge of liquid.
The documents DE 2926691 C2 and U.S. Pat. No. 4,406,170 A, the entire contents of which are incorporated herein by reference, describe means for relocating the fixture body in the housing. These comprise a draw-up lever, connected to the fixture body and projecting out of the housing through a straight slit, for aspirating liquid into the syringe by moving the fixture body away from the fixture. They further comprise a tooth bar pawl device for stepwise piston forward movement by a reciprocating dosing lever. A pivotal pawl is bearing mounted on the dosing lever. The tooth bar is connected to the fixture body and arranged in the pivot region of the pawl. An adjustably movable covering more or less covers up the tooth row on the tooth bar, in order to limit the engagement of the pawl into the tooth bar when the dosing lever is swung. Further, the tooth bar is designed with a contour, by which the covering can be moved away from the tooth bar when the piston is in an advanced position, so that it prevents the pawl from engaging with those teeth of the tooth bar that are not covered. Through this, it is prevented that a residual amount is discharged from the syringe, which is smaller than the dosing amount which is to be discharged in each dosing step.
Further developments of the means for detachably holding the syringe are described in the documents EP 0656229 B1 and U.S. Pat. No. 5,620,660 A, the entire contents of which are incorporated herein by reference. The documents EP 1724020 B1 and U.S. Pat. No. 7,731,908 B2, the entire contents of which are incorporated herein by reference, describe a further development of the holding devices which permits to detach the syringe from the pipette using only one hand.
The documents EP 0657216 B1 and U.S. Pat. No. 5,620,661 A, the entire contents of which are incorporated herein by reference, describe such a pipette with a sensor for gauging protrusions and deepenings on the syringe flange and associated syringes. The sensor serves to determine the size of the utilized syringe. On the basis of the set step width, an electronics determines the amount of liquid that is discharged in each discharging step. This is indicated on a display.
Further developments of the means for relocating the fixture body are described in the documents DE 4437716 C2, EP 0679439 B1 and U.S. Pat. No. 5,591,408 A, the entire contents of which are incorporated herein by reference. According to EP 0679439 B1 and U.S. Pat. No. 5,591,408 A, a repeater pipette has a constant-step equipment, which fixes the size of the first step for relocating the fixture body for the actuation portion of the syringe piston towards the cylinder fixture for the syringe cylinder to a constant value, which is independent from the setting of the subsequent step sizes. Through this constant reverse stroke in the relocation of the fixture body back to the cylinder fixture after drawing up liquid, a clearance between pipette and syringe is overcome which would compromise the dosing accuracy.
The known manually driven repeater pipettes have the disadvantage that the user must look up in the manual the maximum number of dosing steps possible without refilling the syringe at completely filled syringe with the set dosing volume after performing a reverse stroke, or he/she must determine it by experiment. Moreover, the user must count the performed dosing steps when he/she wants to know the number of further dosing steps which are still possible with the remaining liquid in the syringe. This is awesome and prone to errors notably at small dosing step sizes. In addition, it is disadvantageous that no clearance compensation is ensured in the known repeater pipettes, when at only partial filling of the syringe by means of the pipette, the delivery of the liquid from the syringe is controlled by the pipette. The accuracy of the initial dosings is impaired through this.
The document U.S. Pat. No. 6,254,832 B1, the entire contents of which is incorporated herein by reference, describes a portable, microprocessor controlled pipette with an electric power supply. The pipette has a piston in a cylinder, which is driven by an electric step motor. The pipette can be equipped with tubular pipette tips which can be plugged up onto a shaft at the bottom end of the pipette. By means of entry keys, the dispensing volume can be set which is indicated on a display. By actuating a further key, even the number of dispensing steps can be indicated which the pipette can perform with the set dispensing volume. In the dispensing mode, the number of metering steps which is still possible with the residual amount of sample liquid in the pipette tip is indicated after each dispensing step.
Starting from this, the present invention is based on the goal to provide a method for dosing a liquid using a pipette and a syringe, and a pipette for operating a syringe for dosing a liquid having more favourable utilization properties.